1. Field of the Invention
The present invention relates to a resonator using piezoelectric/anti-piezoelectric effects of a piezoelectric layer as well as a resonance phenomenon of a bulk acoustic wave (hereinafter referred to as a “film bulk acoustic wave resonator”), a film bulk acoustic wave resonator filter using such a resonator, and a fabrication method thereof.
2. Description of the Related Arts
In general, a film bulk acoustic wave resonator includes a resonance part having a piezoelectric layer, and first and second metal films, each being obtained by a film forming apparatus, in which the first and second metal films interpose at least part of the piezoelectric layer from above and below. The first metal film serves as an upper electrode and the second metal film serves as a lower electrode. The piezoelectric layer is polarized in its thickness direction. An AC electric field generated by an AC voltage applied between the upper electrode and the lower electrode causes an acoustic wave (i.e., expansion and contraction in the thickness direction of the piezoelectric layer) under piezoelectric/anti piezoelectric effects.
The resonance part composed of the piezoelectric layer, upper electrode, and lower electrode is vertically interposed by acoustic insulators to confine bulk acoustic waves inside the piezoelectric layer. Because the interface between solid and gas (or vacuum) functions as an efficient acoustic wave insulator, an FBAR (Film Bulk Acoustic wave Resonator) having gas (or vacuum) atmosphere above and below is widely known. The basic configuration of a film bulk acoustic wave resonator is disclosed in Japanese Patent Application Laid-Open No. 2006-295380 and 1994 IEEE International Frequency Control Symposium, pp. 135-138, where a resonance part is supported on a hollow by a support layer. In addition, Japanese Patent Application Laid-Open No. 2006-352854 disclosed a resonator configuration where a resonance part is supported on a hollow by a piezoelectric layer.